It is well known that gas phase exothermic catalytic reactions present a number of difficulties when performed in large reactors on an industrial scale. The main problem is how to dissipate the heat released as the reagents are converted into products.
In the prior art it is known that fluid bed reactors have been developed in order to overcome that problem. However they have serious drawbacks, e.g. difficulty in scaling up from the pilot stage to an industrial reactor, catalyst losses due to carry over from the reactors, and low selectivity due to the back mixing regime of the fluid bed. Moreover sometimes it is not possible to have the catalyst suitable for a specific reaction in powder form and with such mechanical properties as are appropriate for a fluid bed reactor.
It is also known from the prior art how to carry out exothermic reactions recycled in fixed bed reactors packed with catalysts in pellet form. However hot spots are present in the catalytic bed, as it is difficult to remove the reaction heat. Hot spots lead to catalyst deactivation and to a decrease in selectivity. As a further disadvantage of the packed bed reactor, the pressure drop is typically very large across the length of the reactor. In an industrial plant such a pressure drop requires that the reacting gases should be compressed to high pressure. This involves a high energy consumption. Further, the unreacted gases have to be compressed before they are used in the reactor.
The prior art does not teach how to carry out an exothermic reaction in a fixed bed reactor operated in a nearly isothermal mode, and achieving high selectivity and very low pressure drop. In industrial practice, in order to reduce the hot spot temperatures with the pellet catalyst beds, a series of complicated measures must be adopted, including narrow tubes, distributed feeds along the reactor, multiple reactors, and multiple layers of catalysts with different activities and different levels of inert dilutions. All of these measures result in more expensive industrial plants and complex process operations; in any case the hot spot temperatures remain high.
Operation of endothermic reactions in packed-bed tubular catalytic reactors also involves difficulties associated with the efficient and uniform transport of the heat of reaction from the reactor tube walls to the catalytically active material, resulting in the formation of cold spots.
In order to overcome the above disadvantages, U.S. Pat. No. 5,099,085 describes the use of honeycomb monolithic catalyst supports (instead of conventional pellets) for exothermic selective chlorination and/or oxychlorination reactions in multitubular reactors with a fixed-bed arrangement of the catalyst. The materials used for preparing the monolith support are activated alumina, aluminum silicate, silica gel, titanium oxide, silicon carbide or mixtures of said materials, or sintered ceramics such a α-Al2O3. Mullite and cordierite are preferred. With the ceramic monolith supports described in this patent and with the related process engineering measures it is possible to reduce the pressure drop, to suppress as far as possible the formation of hot spots and to increase the selectivity in the target products. The ceramic monolith supports have a length from a few centimeters up to about 20 cm and the cross-sectional size, corresponding to the diameter of the reactor tubes, is usually smaller than 20 to 50 mm. The individual catalyst modules (monoliths) are spaced from each other by glass spheres, having a diameter of 3 to 6 mm. From the industrial point of view the spherical packing for spacing makes difficult and complicated the loading of the reactor tubes with the monoliths. In addition, the short monolith length makes the loading operation even more difficult. A further disadvantage of the process of said patent resides in the fact that the hot spot is reduced in respect to the traditional fixed bed catalyst, but it is still too high. In order to obtain this reduced hot spot the reacting gases have to be introduced at different points of the reactor (see FIG. 1 of said patent) thereby creating a multireactor system in order to get better thermal control. Experiments carried out by the present Applicant using the monoliths according to said U.S. patent in a single reactor and feeding the reactants at one point, i.e. using a single reactor, have shown that the hot spot is too high and the selectivity too low for an industrial application (see comparative example).